Microanatomy of the dopaminergic system in the rainbow trout retina

We have investigated the morphology of dopaminergic interplexiform cells as well as the distribution of two classes of dopamine receptors in the retina of the rainbow trout. Interplexiform cells were visualized using an antiserum against tyrosine hydroxylase and PAP immunocytochemistry. In whole amounts, these cells have a density of between 91 and 182 cells per mm2 with highest values in the lower temporal quadrant. Their cell bodies lie at the inner margin of the inner nuclear layer with only 12-17 cells per retina displaced to the ganglion cell layer. There are three levels of stratification in the inner plexiform layer, one at the distal and proximal borders respectively, and one in the middle. They arise mostly from a radially oriented, stout primary dendrite. Tangential processes are about 1 micron in diameter and show a number of varicosities. The density of processes is greatest in sublayer 5, but no major difference in the general organization is apparent between the three sublayers. In the outer retina, there are two levels of dense ramification confined to the layer of horizontal cells. Light and electron microscopic analysis shows synaptic input to horizontal cells, but not to photoreceptors. The distribution of D1 receptors was assessed by studying the binding pattern of a specific, fluorescent-labelled antagonist, SCH 23390, in unfixed frozen sections. We found displaceable binding in the inner and outer plexiform layers and in the region of horizontal cell perikarya. We used an anti-peptide antibody directed to an extracellular domain of the rat D2 receptor and a fluorescent secondary antiserum to study the localization of D2 receptors. In addition to marked label in both plexiform layers, the outer, and especially the inner segments of rods and cones show specific immunoreactivity. In addition, there is distinct label at the level of the horizontal cell bodies; in the inner retina, specific fluorescence is found in somata of some amacrine cells. The significance of the connectivity pattern and the distribution of the two receptor types is discussed with respect to the role of dopamine in controlling adaptational processes in the outer retina, such as retinomotor movements and changes in horizontal cell morphology and physiology.